Field of the Invention
The present invention generally relates to tuning-fork type vibratory gyros, and more particularly to a tuning-fork type vibratory gyro having a piezoelectric substance.
A gyroscope has been used to identify the current position of a vehicle such as an airplane, a ship or a satellite. Recently, a gyroscope has been applied to devices for personal use, such as car navigation and vibration detection in video cameras and still cameras.
A conventional coma gyro detects an angular velocity by utilizing a principle in which a rotating coma (disk) continues to rotate without any change of the attitude thereof while keeping the rotation axis even when a device equipped with the coma gyro is tilted. Recently, an optical type gyro and a piezoelectric type gyro have been developed and reduced to practical use. The principles of the piezoelectric type gyro were proposed around 1950. Various piezoelectric type gyros having, for example, a tuning fork, a cylinder or a semi-spherical member have been proposed. Recently, a vibratory gyro having a piezoelectric member has been in practical use. Such a vibratory gyro has less measurement sensitivity and precision than those of the coma gyro and the optical gyro, but has advantages in terms of size, weight and cost.
FIG. 1 shows a tune fork type vibratory gyro utilizing a piezoelectric single crystal, as disclosed in U.S. Pat. No. 5,329,816. The vibratory gyro shown in FIG. 1 (which is also referred to as a gyro element) includes a piezoelectric single crystal having two arms 10 and 12 and a base 14 supporting the arms 10 and 12. The arms 10 and 12 and the base are integrally formed. A drive electrode 18 for driving a tuning-fork vibration is provided on the arm 12, while a detection electrode 16 for detecting the angular velocity is provided on the arm 10. In the following description, the surface of the gyro appearing in FIG. 1 is referred to as a front surface, while the surface opposite to the front surface is referred to as a back surface. The drive electrode 18 has two electrode portions provided on the front surface of the gyro.
FIG. 2 shows a tune fork type vibratory gyro having a different electrode arrangement from that of the gyro shown in FIG. 1. Such a gyro is disclosed in, for example, U.S. Pat. No. 5,251,483. In FIG. 2, the arm 10 has the detection electrode 16 and the drive electrode 18, and similarly the arm 12 has the detection electrode 16 and the drive electrode 18. The detection electrodes 16 are located closer to the free ends of the arms 10 and 12 than the base 14. In an electrode arrangement shown in FIG. 3, the detection electrodes 16 are located closer to the base 14 than the free ends of the arms 10 and 12.
The capacitance ratios of the gyros shown in FIGS. 1, 2, and 3 are provided in these figures.
However, the gyros shown in FIGS. 1, 2 and 3 have the following respective disadvantages.
The gyro shown in FIG. 1 has the electrode arrangement in which the detection electrode 16 is provided symmetrically with the drive electrode 18. Hence, the capacitance ratios with respect to the drive electrode 18 and the detection electrode 16 are small. However, an unwanted vibration such as a curvature movement is output.
This disadvantage will now be described in detail with reference to FIGS. 4A through 4D. FIG. 4A is a perspective view of the gyro shown in FIG. 1 in which an unwanted vibration is illustrated. FIG. 4B is a side view of the gyro shown in FIG. 4A, FIG. 4C illustrates the unwanted vibration. FIG. 4D shows the electric field caused in the arms 10 and 12 by the unwanted vibration. The electrodes are omitted in FIGS. 4A through 4C. In FIG. 4D, the electrodes with no hatching are at an identical potential, and the electrodes with hatching are at another identical potential. Since the detection electrode 16 is provided on the arm 10 only, the potential difference generated by the electric field shown in FIG. 4D develops. The above potential difference serves as noise, which degrades the detection accuracy. Further, the unwanted vibration may include a torsional vibration, which is a factor causing a temperature drift. Furthermore, a leakage output may occur due to a mechanical coupling and/or electrostatic coupling between the detection-side arm and the drive-side arm.
In the electrode arrangement shown in FIG. 2, a reduction in the drive voltage can be realized because the capacitance ratio with respect to the drive electrodes 18 is small. Further, the detection electrodes 16 are provided on the arms 10 and 12, so that the unwanted vibration can be canceled and the leakage output is small. However, the capacitance ratios obtained at the free ends of the arms 10 and 12 are as large as approximately twenty times those obtained at the root portions thereof, and the sensitivity is thus small. Furthermore, the wiring lines extending from the detection electrodes 16 and the drive electrodes 18 are complex and the productivity is not high because the detection electrodes 16 and the drive electrodes 18 are provided on the arms 10 and 12.
The electrode arrangement shown in FIG. 3 enables high sensitivity because the capacitance ratio with respect to the detection electrodes 16 is small. However, a high drive voltage is required because the capacitance ratio with respect to the drive electrodes 18 is high. Furthermore, the wiring lines extending from the detection electrodes 16 and the drive electrodes 18 are complex and the productivity is not high because the detection electrodes 16 and the drive electrodes 18 are provided on the arms 10 and 12.